(A) Field of the Invention
The present invention relates to an overlay mark for checking alignment accuracy, and more particularly, to an overlay mark for aligning different layers on a semiconductor wafer.
(B) Description of the Related Art
Nowadays, the semiconductor devices and integrated circuits include multi-layer structures having dimensions smaller than one micrometer. Proper alignment of different layers is essential for proper performance of the fabricated semiconductor devices and circuits. Overlay measurements are routinely performed to verify the proper alignment. Lack of proper alignment typically results in erroneous measurement of specification and fails to meet the requirements of the client.
Overlay measurements optically measure the relative positions of the overlay marks on different layers of a structure on the semiconductor wafer. More particularly, a rectangular overlay mark is formed on each layer of the structure. When two rectangular overlay marks on two consecutive layers are centered with respect to each other, the two layers are properly aligned to each other. The rectangular overlay marks are also referred to as bar-in-bar patterns.
FIG. 1 is a top view of an overlay mark 10 for checking alignment accuracy according to the prior art. As shown in FIG. 1, the overlay mark 10 includes four inner bars 12 and four outer bars 14 on a semiconductor wafer 16, wherein the outer bars 14 represent the pattern of the pre-layer, and the inner bars 12 represent the pattern of the present layer, such as a photoresist layer. Each bar is a side of the rectangle and the sides are not connected, wherein the rectangle formed by the outer bars 14 encloses the rectangle formed by the inner bars 12.
In the process of checking alignment accuracy, a checking beam scans in a scanning direction 18 across the two outer bars 14 and two inner bars 12, as shown in FIG. 1. After scanning, the signals of the actual position representing the outer bars 14 and the inner bars 12 are read. In addition, the differences (overlay error) between the mean values of the positions of the two outer bars 12 and 14 are calculated. If the overlay error is larger than an acceptable deviation value, the alignment between the pattern of the pre-layer and that of the photoresist layer will not meet the accuracy requirements. Under this situation, the photoresist layer has to be removed, and a second photolithography process has to be repeated until the overlay error is smaller than the acceptable deviation value.
FIG. 2 is a schematic diagram showing the application of the bar-in-bar pattern for checking alignment accuracy between three layers on a semiconductor wafer 26 according to the prior art. As shown in FIG. 2, to check alignment accuracy between three layers, the prior art technology used two separate bar-in-bar patterns 20A and 20B. The bar-in-bar pattern 20A is formed of four inner bars 32A and four outer bars 34, while the bar-in-bar pattern 20B is formed of four inner bars 32B and four outer bars 36. The outer bars 34 represent the pattern of the first pre-layer, the inner bars 32A and 32B represent the pattern of the second pre-layer, and the outer bars 36 represent the pattern of the present layer, such as a photoresist layer.
According to the prior art, the bar-in-bar patterns 20A is used for checking alignment accuracy between the first pre-layer and the second pre-layer, and the bar-in-bar patterns 20B is used for checking alignment accuracy between the second pre-layer and the present layer. Since there are two set of inner bars 32A, 32B required to connect the outer bars 34, 36, respectively, the area cost is very high for the two bar-in-bar patterns 20A, 20B positioned on different areas of the semiconductor wafer 26. As the design rule shrinks and the fabrication of the integrated circuits tends to use multi-layer design, the area cost issue of the bar-in-bar patterns becomes seriously high.
Additionally, since the four outer bars 34 and the four outer bars 36 do not possess the bar-in-bar relationship, the alignment accuracy between the first pre-layer and the present layer can not be checked by direct measuring the positions of the four outer bars 34 and the four outer bars 36. Instead, the alignment accuracy between the first pre-layer and the present layer can only be obtained from the alignment accuracy of the bar-in-bar patterns 20A and 20B that requires to measure all positions of the inner bars 32A, 32B and the outer bars 34, 36 of different bar-in-bar patterns. However, this will increase the measuring time and the deviation between the bar-in-bar patterns 20A and 20B, and will certainly influence the checking result between the first pre-layer and the present layer.